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><A
NAME="TUTORIAL-FK"
>3.3. Foreign Keys</A
></H1
><P
>    Recall the <CODE
CLASS="CLASSNAME"
>weather</CODE
> and
    <CODE
CLASS="CLASSNAME"
>cities</CODE
> tables from <A
HREF="tutorial-sql.html"
>Chapter 2</A
>.  Consider the following problem:  You
    want to make sure that no one can insert rows in the
    <CODE
CLASS="CLASSNAME"
>weather</CODE
> table that do not have a matching
    entry in the <CODE
CLASS="CLASSNAME"
>cities</CODE
> table.  This is called
    maintaining the <I
CLASS="FIRSTTERM"
>referential integrity</I
> of
    your data.  In simplistic database systems this would be
    implemented (if at all) by first looking at the
    <CODE
CLASS="CLASSNAME"
>cities</CODE
> table to check if a matching record
    exists, and then inserting or rejecting the new
    <CODE
CLASS="CLASSNAME"
>weather</CODE
> records.  This approach has a
    number of problems and is very inconvenient, so
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> can do this for you.
   </P
><P
>    The new declaration of the tables would look like this:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE cities (
        city     varchar(80) primary key,
        location point
);

CREATE TABLE weather (
        city      varchar(80) references cities(city),
        temp_lo   int,
        temp_hi   int,
        prcp      real,
        date      date
);</PRE
><P>

    Now try inserting an invalid record:

</P><PRE
CLASS="PROGRAMLISTING"
>INSERT INTO weather VALUES ('Berkeley', 45, 53, 0.0, '1994-11-28');</PRE
><P>

</P><PRE
CLASS="SCREEN"
>ERROR:  insert or update on table "weather" violates foreign key constraint "weather_city_fkey"
DETAIL:  Key (city)=(Berkeley) is not present in table "cities".</PRE
><P>
   </P
><P
>    The behavior of foreign keys can be finely tuned to your
    application.  We will not go beyond this simple example in this
    tutorial, but just refer you to <A
HREF="ddl.html"
>Chapter 5</A
>
    for more information.  Making correct use of
    foreign keys will definitely improve the quality of your database
    applications, so you are strongly encouraged to learn about them.
   </P
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